1. Field of the Invention
The present invention relates to an optical apparatus having an automatic focusing detection means. More particularly, the invention relates to an optical apparatus with an automatic focusing detection means suitably adaptable to a video, slide projector for movie films, overhead projector (OHP), photographic apparatus for blackboard descriptions, or the like, capable of projecting the projection image on a screen in an excellent condition by obtaining ranging information (distance information) in a plurality of areas on the aforesaid screen when a projecting image source such as a projection image or liquid crystal light bulb is projected on the screen by a projecting system.
2. Related Background Art
Traditionally, among those projectors which project the image of a projecting image source on a screen with a projecting optical system, there has been proposed a projector provided with an automatic focusing detection means (hereinafter referred to as AF means) thereby projecting an in focus image of a projecting image source on a screen.
FIG. 5 is a diagrammatical view showing the structure of the principal part of a projector having a conventional AF means. In FIG. 5, a reference numeral 201 designates a projector and 202, an AF means. The projector 201 in FIG. 5 projects the image on a projection image display 50 with a liquid crystal light bulb and others illuminated by a light beam from an illuminating light source 12 on a screen 1 via a projecting optical system 47.
At this juncture, the focusing adjustment, i.e., the AF operation, of a projecting system 203, whereby the image on the projection image display 50 should be focused on the screen 1 by the AF means 202, is performed as described below in accordance with a trigonometrical range finding method disclosed in Japanese Patent Laid-Open Application No. 60-256114, for example.
At first, by a controller 32, a floodlight driving signal 33 is inputted into a driving circuit 34. The driving circuit 34 generates a driving current 35 to apply to a floodlight element 36 like a LED. The floodlight element 36 emits infrared light. The infrared light 38 is projected on a screen 1 by a floodlight lens 37. The infrared light 38 projected onto the screen 1 is reflected. The reflected light 39 enters the light receiving element 41 comprising a two-piece sensor, position sensor and others through a light receiving lens 40. The light beam 39 at this juncture is converted into an electrical signal 42 corresponding to the incident position of the light receiving element 41. The electrical signal 42 is processed by a signal processing circuit 43 and is inputted into the controller 32 as a focus error signal 44.
The driving system 46 which comprises the driving circuit, driving actuator, and others drives a focus lens group 47 along the optical axis in response to a signal 45 from the controller 32, which indicates the appropriate adjustment (focusing) for the projecting system 203, thus performing the focusing adjustment.
On the other hand, when an image should be projected on a screen by a projector, the image is projected on the screen 1 provided that the optical axis 103a of the projecting optical system 103 is substantially perpendicular to the central region 1a of the screen 1 as shown in FIG. 3, and an image 81 of the oblong projecting image source, for example, becomes an oblong shape as shown in FIG. 3. In this case, the screen 1 is identical to the ideal projection plane.
Therefore, if the focusing condition in one portion of the projected image on the screen 1 is detected, it is possible to obtain a projected image on the entire area of the screen, the focusing of which has been performed as desired.
In contrast, if the center of the screen is located at a position higher than the projector, for example, the optical axis 103a of the projecting optical system enters on a slant at a large angle with respect to the central region 1a of the screen 1 as shown in FIG. 4. 10 Consequently, as shown in FIG. 4, the oblong projection image source 50 is distorted from the oblong shape to be a projected image of a trapezoid shape on the screen 1. Furthermore, the projected image 82 is out of focus but only in a portion thereof. Because of this, even if an AF means is provided for the projector, only a part of the projected image 82 on the screen 1 can be focussed by the use of the AF means, and there is a problem that the projected image of an excellent quality all over the screen cannot be obtained.